Charged particle beams, laser beams, and neutral particle beams are used in a variety of microfabrication applications, such as fabrication of semiconductor circuitry and microelectromechanical assemblies. The term “microfabrication” is used to include creating and altering structures having dimensions of tens of microns or less, including nanofabrication processes. “Processing” a sample refers to the microfabrication of structures on that sample. As smaller and smaller structures are fabricated, it is necessary to direct the beam more precisely. It has been found, however, that the impact point of a beam on a sample tends to drift over time. That is, when the operator instructs the system to position the beam at point P1, the beam actually ends up at position P2 a short time later. The difference between the coordinates of the points P1 and P2 is referred to as the beam drift. The drift can be caused by mechanical or thermal instabilities that cause slight movement of the stage on which the sample is positioned or of the elements that generate and focus the beam. While the drift is small, it becomes more significant as smaller structures are fabricated.
One method of accurately positioning a beam is to mill a fiducial, that is, a reference mark, and position the beam relative to the fiducial. The term fiducial is used broadly to include any type of reference mark. A beam is initially directed to image a fiducial and an initial offset to the desired location is determined. Subsequently, the beam is periodically directed to image the fiducial and the positioning of the beam to the desired location is corrected by determining an offset between the observed coordinates of the fiducial and the original coordinates of the fiducial. The offsets are then added to the beam positioning instructions so that the beam ends up at the desired location. Repeated scanning of a fiducial with an ion beam, however, degrades the fiducial, making it less useful for precise positioning. Thus there is a need for a system which periodically scans a fiducial with a beam which does not damage the fiducial, while also using a beam to do microfabrication at a desired location.
One aspect of semiconductor manufacturing that requires accurate beam positioning is the extraction of thin samples for transmission electron microscopy. Such samples are used for monitoring the semiconductor fabrication process. A thin, vertical sample, referred to as a lamella, is extracted to provide a vertical cross section of the structure.
A transmission electron microscope (TEM) allows an observer to image extremely small features, on the order of nanometers or smaller. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the sample are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface. This technique can allow an observer to image features of sizes below one nanometer.
For both TEM and STEM, beam positioning while preparing the thin sample is important, because the beam must not etch away the feature of interest, yet the sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples are typically less than 100 nm thick.
One technique to cut a sample from a substrate or bulk sample without destroying or damaging surrounding parts of the substrate is referred to as a “lift-out” technique. A focused ion beam (FIB) is typically used to free the sample. A lift-out technique is useful in analyzing the results of processes used in the fabrication of integrated circuits, as well as analyzing materials in the physical or biological sciences.
FIGS. 1-3 illustrate a commonly used sample preparation technique. A protective layer 100 of a material such as tungsten is deposited over the area of interest on a sample surface 102 using electron beam or ion beam assisted deposition. Fiducials 104 are milled near the region of interest to serve as reference markers for aligning the focused ion beam that will be used to cut the sample. Next, as shown in FIG. 2, a focused ion beam using a high beam current with a correspondingly large beam size is used to mill rectangles 202 and 204, respectively in front of and behind the region of interest, leaving a thin vertical sample section, lamella 206, that includes a vertical cross section of the area of interest.
As shown in FIG. 3, once the specimen reaches the desired thickness, the stage is tilted and a U-shaped cut 302 is made at an angle partially along the perimeter of the lamella 206, leaving the lamella hanging by tabs 304 at either side at the top of the sample. The sample section is then further thinned using progressively finer beam sizes. Finally, a probe (not shown) is attached to the lamella 206 and the tabs 304 are cut to completely free the thinned lamella 206.
Because of the precision required for each operation, it is necessary that the beam be placed accurately for each cut, particularly for the final thinning operation. This can be a problem because the beam position tends to drift over time as described above. One solution is to remove most of the material required to extract the sample and then to correct for drift by imaging a fiducial just before a final cut is made. Although the time required to make the final cut is not as long as the time required to make the first cut, the beam still drifts while making the final cut, reducing the milling accuracy.
U.S. Pat. No. 6,521,890 for “Focused Ion Beam Machining Method and Focused Ion Beam Machining Apparatus” to Ishitani et al. teaches using secondary ion microscopy to form images of a fiducial in the field of the view of the ion beam at prescheduled intervals and then to adjust the ion beam based on comparing each image to the previous image. Ishitani et al. explains that use of the previous image for comparison compensates for change in shape of the fiducial caused by the ion beam. As the fiducial degrades, however, it becomes a less precise reference.
U.S. Pat. No. 5,315,123 for “Electron Beam Lithography Apparatus” to Itoh et al. teaches correction of electron beam drift in electron beam lithography. The drift of an electron beam during the lithography writing process is determined by measuring a reference mark on the stage and the position of the beam is corrected for the beam drift while the beam is blanked as the stage is moved between circuit patterns. The described technique uses a single reference mark for multiple circuit patterns and the stage moves between corrections, which can introduce errors into the positioning. The technique is also limited to imaging fiducials outside the area being processed by the electron beam.